Hey all - new here. I was just wondering what these 3 slots in the back of the sys3 and sys4 are for if anything. Business cards slots? Do they work with certain inserts? And what about the notches on either side? Maybe I’m thinking too much into this ha. Anyways thanks for your help!

Probably for attachments. Tanos makes Systainers and they seem to have quite a few end-customers beyond Festool and I'm guessing those are for dividers or in place for even some kind of interior lid accessory perhaps.

Those slots hold a few different add-on Systainer items such as the strap that comes with the CT Sys or the separate holders that hold items such as levels on the exterior of Systainers. They have the same widths as the front of the Systainer where the ID cards would normally go. Can't find the P/Ns at the moment. I've also seen the external clips used in those slots to hold the boxes such as the ones that come with the parts organizer Systainer.